Did you encounter those two cultists previously and send them back to the Goblin camp? They presumably told the Goblins about the owlbear who killed their companion, resulting in the goblins having gone to the cave to kill her and capturing the cub. Maybe if the cultists aggroed and were killed instead during that encounter, the owlbear cub and its mom were still safe in their cave, hence the chicken substitution?
Only way to test this hypothesis is to go to the save right before encountering those cultists on the trail.
